$XMLRequest = <<<XMLSCR
    <?xml version="1.0" encoding="UTF-8"?>
    <soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
      <soap12:Header>
        <AuthenticationHeader xmlns="http://gso.com/GsoShipWS">
          <UserName>username</UserName>
          <Password>password</Password>
        </AuthenticationHeader>
      </soap12:Header>
      <soap12:Body>
        <GetServiceTypes xmlns="http://gso.com/GsoShipWS">
          <GetServiceTypesRequest>
            <AccountNumber>12345</AccountNumber>
          </GetServiceTypesRequest>
        </GetServiceTypes>
      </soap12:Body>
    </soap12:Envelope>
    XMLSCR;

I've also try with passing array

$xml_array = array(
    'GetServiceTypesRequest' => array(
        'AccountNumber' => 12345,
    )
);

$xml_post_string = '<?xml version="1.0" encoding="utf-8"?>
    <soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
      <soap12:Header>
        <AuthenticationHeader xmlns="http://gso.com/GsoShipWS">
          <UserName>username</UserName>
          <Password>password</Password>
        </AuthenticationHeader>
      </soap12:Header>
      <soap12:Body>
        <GetServiceTypes xmlns="http://gso.com/GsoShipWS">
          <GetServiceTypesRequest>
            <AccountNumber>12345</AccountNumber>
          </GetServiceTypesRequest>
        </GetServiceTypes>
      </soap12:Body>
    </soap12:Envelope>';

  // data from the form, e.g. some ID number



$headers = array(
                    "POST /GSOShipWS1.0/GSOShipWS.asmx HTTP/1.1",
                    "Host: wsa.gso.com",
                    "Content-Type: application/soap+xml; charset=utf-8",
                    "Content-length: ".strlen($xml_post_string),
                ); //SOAPAction: your op URL

        $url = $soapUrl;

        // PHP cURL  for https connection with auth
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($ch, CURLOPT_URL, $url);
        //curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_USERPWD, username.":".password); // username and password - declared at the top of the doc
        curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_ANY);
        curl_setopt($ch, CURLOPT_TIMEOUT, 10);
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $xml_post_string); // the SOAP request
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

        // converting
        $response = curl_exec($ch);